poshaughnessy-blueimp-load-image

JavaScript Load Image

A JavaScript library to load and transform image files.

Table of contents

• Demo
• Description
• Setup
• Usage
• Image loading
• Image scaling
• Requirements
• API
• Options
• Meta data parsing
• Exif parser
• License
• Credits

Demo

JavaScript Load Image Demo

Description

JavaScript Load Image is a library to load images provided as File or Blob objects or via URL.
It returns an optionally scaled and/or cropped HTML img or canvas element via an asynchronous callback.
It also provides a method to parse image meta data to extract Exif tags and thumbnails and to restore the complete image header after resizing.

Setup

Include the (combined and minified) JavaScript Load Image script in your HTML markup:

<script src="js/load-image.all.min.js"></script>

Or alternatively, choose which components you want to include:

<script src="js/load-image.js"></script>
<script src="js/load-image-orientation.js"></script>
<script src="js/load-image-meta.js"></script>
<script src="js/load-image-exif.js"></script>
<script src="js/load-image-exif-map.js"></script>

Usage

Image loading

In your application code, use the loadImage() function like this:

document.getElementById('file-input').onchange = function (e) {
    loadImage(
        e.target.files[0],
        function (img) {
            document.body.appendChild(img);
        },
        {maxWidth: 600} // Options
    );
};

Image scaling

It is also possible to use the image scaling functionality with an existing image:

var scaledImage = loadImage.scale(
    img, // img or canvas element
    {maxWidth: 600}
);

Requirements

The JavaScript Load Image library has zero dependencies.

However, JavaScript Load Image is a very suitable complement to the Canvas to Blob library.

API

The loadImage() function accepts a File or Blob object or a simple image URL (e.g. 'https://example.org/image.png') as first argument.

If a File or Blob is passed as parameter, it returns a HTML img element if the browser supports the URL API or a FileReader object if supported, or false.
It always returns a HTML img element when passing an image URL:

document.getElementById('file-input').onchange = function (e) {
    var loadingImage = loadImage(
        e.target.files[0],
        function (img) {
            document.body.appendChild(img);
        },
        {maxWidth: 600}
    );
    if (!loadingImage) {
        // Alternative code ...
    }
};

The img element or FileReader object returned by the loadImage() function allows to abort the loading process by setting the onload and onerror event handlers to null:

document.getElementById('file-input').onchange = function (e) {
    var loadingImage = loadImage(
        e.target.files[0],
        function (img) {
            document.body.appendChild(img);
        },
        {maxWidth: 600}
    );
    loadingImage.onload = loadingImage.onerror = null;
};

The second argument must be a callback function, which is called when the image has been loaded or an error occurred while loading the image. The callback function is passed one argument, which is either a HTML img element, a canvas element, or an Event object of type error:

var imageUrl = "https://example.org/image.png";
loadImage(
    imageUrl,
    function (img) {
        if(img.type === "error") {
            console.log("Error loading image " + imageUrl);
        } else {
            document.body.appendChild(img);
        }
    },
    {maxWidth: 600}
);

Options

The optional third argument to loadImage() is a map of options:

• maxWidth: Defines the maximum width of the img/canvas element.
• maxHeight: Defines the maximum height of the img/canvas element.
• minWidth: Defines the minimum width of the img/canvas element.
• minHeight: Defines the minimum height of the img/canvas element.
• sourceWidth: The width of the sub-rectangle of the source image to draw into the destination canvas.
  Defaults to the source image width and requires canvas: true.
• sourceHeight: The height of the sub-rectangle of the source image to draw into the destination canvas.
  Defaults to the source image height and requires canvas: true.
• top: The top margin of the sub-rectangle of the source image.
  Defaults to 0 and requires canvas: true.
• right: The right margin of the sub-rectangle of the source image.
  Defaults to 0 and requires canvas: true.
• bottom: The bottom margin of the sub-rectangle of the source image.
  Defaults to 0 and requires canvas: true.
• left: The left margin of the sub-rectangle of the source image.
  Defaults to 0 and requires canvas: true.
• contain: Scales the image up/down to contain it in the max dimensions if set to true.
  This emulates the CSS feature background-image: contain.
• cover: Scales the image up/down to cover the max dimensions with the image dimensions if set to true.
  This emulates the CSS feature background-image: cover.
• aspectRatio: Crops the image to the given aspect ratio (e.g. 16/9).
  Setting the aspectRatio also enables the crop option.
• pixelRatio: Defines the ratio of the canvas pixels to the physical image pixels on the screen.
  Should be set to window.devicePixelRatio unless the scaled image is not rendered on screen.
  Defaults to 1 and requires canvas: true.
• downsamplingRatio: Defines the ratio in which the image is downsampled.
  By default, images are downsampled in one step. With a ratio of 0.5, each step scales the image to half the size, before reaching the target dimensions.
  Requires canvas: true.
• crop: Crops the image to the maxWidth/maxHeight constraints if set to true.
  Enabling the crop option also enables the canvas option.
• orientation: Transform the canvas according to the specified Exif orientation, which can be an integer in the range of 1 to 8 or the boolean value true.
  When set to true, it will set the orientation value based on the EXIF data of the image, which will be parsed automatically if the exif library is available.
  Setting the orientation also enables the canvas option.
  Setting orientation to true alsoe enables the meta option.
• meta: Automatically parses the image meta data if set to true.
  The meta data is passed to the callback as second argument.
• canvas: Returns the image as canvas element if set to true.
• crossOrigin: Sets the crossOrigin property on the img element for loading CORS enabled images.
• noRevoke: By default, the created object URL is revoked after the image has been loaded, except when this option is set to true.

They can be used the following way:

loadImage(
    fileOrBlobOrUrl,
    function (img) {
        document.body.appendChild(img);
    },
    {
        maxWidth: 600,
        maxHeight: 300,
        minWidth: 100,
        minHeight: 50,
        canvas: true
    }
);

All settings are optional. By default, the image is returned as HTML img element without any image size restrictions.

Meta data parsing

If the Load Image Meta extension is included, it is also possible to parse image meta data.
The extension provides the method loadImage.parseMetaData, which can be used the following way:

loadImage.parseMetaData(
    fileOrBlob,
    function (data) {
        if (!data.imageHead) {
            return;
        }
        // Combine data.imageHead with the image body of a resized file
        // to create scaled images with the original image meta data, e.g.:
        var blob = new Blob([
            data.imageHead,
            // Resized images always have a head size of 20 bytes,
            // including the JPEG marker and a minimal JFIF header:
            loadImage.blobSlice.call(resizedImage, 20)
        ], {type: resizedImage.type});
    },
    {
        maxMetaDataSize: 262144,
        disableImageHead: false
    }
);

The third argument is an options object which defines the maximum number of bytes to parse for the image meta data, allows to disable the imageHead creation and is also passed along to segment parsers registered via loadImage extensions, e.g. the Exif parser.

Note:
Blob objects of resized images can be created via canvas.toBlob().

Exif parser

If you include the Load Image Exif Parser extension, the argument passed to the callback for parseMetaData will contain the additional property exif if Exif data could be found in the given image.
The exif object stores the parsed Exif tags:

var orientation = data.exif[0x0112];

It also provides an exif.get() method to retrieve the tag value via the tag's mapped name:

var orientation = data.exif.get('Orientation');

By default, the only available mapped names are Orientation and Thumbnail.
If you also include the Load Image Exif Map library, additional tag mappings become available, as well as two additional methods, exif.getText() and exif.getAll():

var flashText = data.exif.getText('Flash'); // e.g.: 'Flash fired, auto mode',
 
// A map of all parsed tags with their mapped names as keys and their text values:
var allTags = data.exif.getAll();

The Exif parser also adds additional options for the parseMetaData method, to disable certain aspects of the parser:

• disableExif: Disables Exif parsing.
• disableExifThumbnail: Disables parsing of the Exif Thumbnail.
• disableExifSub: Disables parsing of the Exif Sub IFD.
• disableExifGps: Disables parsing of the Exif GPS Info IFD.

License

The JavaScript Load Image script is released under the MIT license.

Credits

• Image meta data handling implementation based on the help and contribution of Achim Stöhr.
• Exif tags mapping based on Jacob Seidelin's exif-js.